Build a terraced end house with an additional unit (GÜ) on your own

There is nothing you can do since the construction obligation is within two years after purchase – and that only ends in November.

But that probably shouldn't be the case. As an owner, you do not have to tolerate such an intervention on your property for such long periods. Especially not when there has been a standstill for months.

The problem is that we are more or less responsible for the construction delay ourselves due to the preliminary injunction.

Here, too, a judge will probably not determine that you are responsible for the delay.

As I would summarize the situation, the middle house was poorly planned and you would have suffered even greater damage if you had not applied for the preliminary injunction.

The standard procedure here would be to set a reasonable deadline for the neighbor to remove the disturbance to your property and refer to § 1004 of the Building Code. If necessary, you can also assert claims for damages under §§ 823 and 249 of the Building Code.

that only ends in November.

I would definitely not wait for this, because this creates dependence on the authorities having to take action, and they usually act very slowly.

Of course, I also understand that this situation is extremely unfortunate for you and that further legal steps can again be stressful and time-consuming.

However, I see that you will not be able to use your garden for a long time otherwise.

After everything you have had to endure with this municipality and this neighbor, I really wish from the bottom of my heart that you can at least finish designing and enjoy your domain.

Well, being right and getting justice are unfortunately two different things

So if there is an online petition for the municipality to buy the middle property, fill it in, and hand it over to you two "Randhäusern" for free along with two garages plus a garden shed as compensation for the stress caused by absolutely stupid planning, then send me the link - I'll sign immediately!

Something on the topic again:

Due to the rather limited space in the garden, we could probably place at most a garden shed (Biohort Avantgarde A5) with internal dimensions of 252 x 172cm.

What do you think? Is that enough for four wheels (currently two large ones and soon two children's bikes) as well as some garden stuff like small battery mowers, two folding chairs, etc.?

Yes. Two bicycle wall mounts for the big wheels (uses the height), place the small ones in between. Biohort also has wall shelves and hooks.

Something relevant again:

Due to the rather cramped conditions in the garden, we could probably at most place a garden shed (Biohort Avantgarde A5) with internal dimensions of 252 x 172 cm.

What do you think? Is that enough for four wheels (currently two large ones and soon two children's bikes) as well as a bit of garden stuff like small cordless mowers, two folding chairs, etc.?

I think you are mistaken. I will list what we have inside over the winter: garden chairs, cushions, sun lounger, grill, bicycles, cordless mower, tires, tools, shovels, hoes, brooms, shelves for stuff. In addition, the children's bikes quickly turn into 26" wheels as they get as big as your bike.